1. Field of the Invention
This invention relates generally to nuclear magnetic resonance (NMR) apparatus and techniques for logging wells. More specifically, the invention relates to antenna designs for NMR well logging apparatus.
2. Background Art
Nuclear magnetic resonance (NMR) logging tools measure the amplitude and the decay constant of an NMR signal from the spin nuclei in earth formation, notably protons that are constituents of both water and hydrocarbons. The initial signal amplitude is a measure of total formation porosity while the time decay, invariably multi-exponential, can be decomposed into a distribution of exponential decays with different transverse relaxation times. The transverse relaxation time, T2, is a measure of spin-spin interaction that provides information on the pore size, type of fluid, and hydraulic permeability of the formation. These parameters are important petrophysical quantities, explaining why NMR logging is popular.
The quality of NMR logs is strongly dependent on the signal to noise ratio, S/N, of the measurement. S/N is determined by the strength of the static magnetic field, the strength of the RF field, and the relative orientation of these two fields in the sensed region. The S/N also depends on the volume of the sensed region. In pulse NMR logging tools, a static magnetic field, B0, along the z-axis, is used to polarize the nuclear spins, causing the individual spins to precess around B0 at the so called Larmor frequency, xcfx89 L. In a typical measurement cycle, the RF field, B1, is used to flip the magnetization to another plane, often perpendicular to the direction of static magnetic filed, to generate an NMR signal in the receiving antenna. The voltage induced at the output terminals of the receiving RF antenna due to a point magnetic dipole, such as a precessing nuclear spin, located at position r, is given by,
V(r, xcfx89)=|xcfx89B1(r, xcfx89).m/|(xcfx89)xe2x80x83xe2x80x83(1)
Where, xcfx89 is the angular precession frequency, B1 is the RF magnetic field generated by the antenna when it is excited by an RF current I(xcfx89), and m is magnetic dipole moment of the nuclear spin. In macroscopic samples where many spins are present, a vector sum of m from all the spins in a unit volume constitutes the nuclear magnetization, M0, which before the application of the RF pulse is aligned with B0. Note that the signal is proportional to the scalar product of m (or for an ensemble of spins, M0) and B1, and thus is a maximum if B1 and M0 are aligned. This is why the RF antenna is designed so as in the sensing region the B1 field is as perpendicular to B0 as possible. This arrangement is to ensure that after the first 90 pulse, the magnetization and the B1 field are aligned. This is one of the conditions for maximizing the signal and increasing the S/N.
Another important parameter affecting the sensitivity of NMR measurement is the volume of sensed region. The total measured signal is the sum of the signal from all the excited spins in the sensed region and is given by:
S=Integral(V(r, xcfx89)dv)xe2x80x83xe2x80x83(2)
Where, V(r, xcfx89), given by Eq. 1 above, is the local NMR signal intensity from the spins in the differential volume element dv centered at r, and the integration is performed over the volume of sensed region.
For a gradient-type NMR logging tool operating at a frequency xcfx89, there are two ways of increasing the volume of sensed region. The first is to increase B1 field strength, which causes the pulse length to decrease, this in turn increases the frequency content of the RF pulse, leading to a thicker sensed region in the radial direction. A second method is to increase the physical dimensions of the antenna, usually the antenna length. For the same antenna design, the length of the antenna is directly proportional to the length of the sensed region. In addition to its effect on the S/N, changing the length of antenna affects the optimum depth of investigation where the antenna is most efficient, the log speed effect, and the vertical resolution, discussed in more detail below.
In addition to increasing the volume of sensed region, a longer antenna is more suitable to transmit and receive for deeper depths of investigations. This is particularly attractive since at deeper depths of investigations the S/N of NMR is inherently lower.
Increasing the antenna length also reduces, but does not eliminate, the speed effect. When the antenna is used to apply a 90 pulse, the magnetization in front of the antenna is rotated into the transverse plane and is ready to be sampled. To sample the magnetization, using a CPMG sequence for example, one uses a series of 180 pulses, that are applied at a later time after the 90 pulse, during this time delay the tool/antenna has moved in the logging direction. At the time of the nth 180 pulse, the tool logging with a speed of v inches/sec, has traveled for (2nxe2x88x921)(xcfx84/2)v inches, where xcfx84 is the time between subsequent 180 pulses. This move causes the 180 pulse to be applied to the formation that is (2nxe2x88x921)(xcfx84/2)v inches shifted relative to the formation that was initially tipped by the 90 pulse. The result is an (2nxe2x88x921)(xcfx84/2)v inch of formation that has not been rotated into the transverse plane, and thus is not sampled properly. At the same time (2nxe2x88x921)(xcfx84/2)v inches of formation in which the spins have been rotated into the transverse plane, fall outside the viewing range of the antenna; these spins are xe2x80x9cleft behindxe2x80x9d. The spins in the xe2x80x9cleft behindxe2x80x9d region do not sense the 180 pulse and do not contribute to the echo intensity. This leads to a loss of signal that is proportional to logging speed. This signal loss is termed herein as the speed effectxe2x80x94it is not present if the tool is stationary. For the same logging speed, as the length of the antenna (L) increases, the relative contribution of the signal from the spins that are left behind, (2nxe2x88x921)(xcfx84/2)v/L, decreases and the speed effect is reduced.
A longer antenna samples a longer section of the formation, thus has a lower vertical resolution. This is a drawback for the measurement and limits the antenna length in NMR logging tools.
A typical antenna in NMR logging tools is oriented along the tool axis (axial direction). Due to the space restrictions in logging, if the antenna is designed for high efficiency (as opposed to high resolution), the axial-dimension of NMR antennas is longer than the tangential- and radial-dimensions. Electrically, these antennas have a radiation pattern that is approximately the same as that of a rectangular loop antenna oriented in the axial-tangential or axial-radial planes, see FIG. 1 for a pad tool example. Similarly, centralized logging tools utilize windings in the axial-tangential plane, shown in FIG. 2. These antennas are designed to have B1 components orthogonal to B0 in the sensed region, as is required by Eq. 1. The length of typical antennas, for example, is 24xe2x80x3, defining the minimum vertical resolution of the tools. These antennas are designed with high S/N, having relatively low vertical resolution, and are termed herein as main or primary coils.
An antenna design is provided utilizing multiple antenna coils to obtain high vertical resolution NMR measurements of earth formations surrounding a borehole. This antenna can be used alone or in conjunction with the primary coil. A primary coil is situated across a longitudinal axis of a magnet. A secondary coil having smaller dimensions (and higher resolution) than the dimensions of the primary coil is also situated along the longitudinal axis of the magnet. The primary and secondary coils can be operated either independently or in combination to obtain NMR signals from a portion of an earth formation.
In a non-overlapping configuration, the secondary coil is spaced apart from the primary coil a distance that minimizes electrical coupling between the coils. If this is the case, the two antennas have to be operated in an active mode, each acting as both a transmitter and a receiver. In another non-overlapping antenna design, the secondary coil is situated in a cross coil configuration having its radiation polarization orthogonal to that of the primary coil, thereby minimizing the separation between the coils while maintaining electrical isolation.
A similar antenna configuration situates either a parallel high resolution coil or an orthogonal cross coil at a location along the magnet that overlaps the primary coil. The secondary coil is situated either partially overlapping or completely embedded within the primary coil. In either embodiment the secondary coil is operated in a receiver mode or a dual transmitter/receiver mode.